The autoregulation mechanisms of the blood pressure of human beings, for example, ensure that the cardiovascular system reacts in an appropriate manner to sudden blood pressure variations (change in position, injury, etc.) such that physical damages due to too high or too low blood pressure are avoided.
Due to the measurement of this autoregulation, it is possible to draw conclusions about the autonomous control of the cardiovascular activity. Applications are for example in the risk assessment of cardiovascular diseases. Specifically the risk of a sudden cardiac death seams to be directly connected to the vagal control of the cardiovascular system.
A method of measuring the autoregulation consists in the measurement of the baroreflex system feedback. In the baroreflex regulation mechanism, the blood pressure is monitored via baroreceptors in the aortic arch and the carotids, and signals are forwarded to the vegetative nervous system in case of an increase or decrease in blood pressure. When the blood pressure increases, the sympathetic nervous system is dampened, and the parasympathetic nervous system is activated. This leads, among others, to a decrease in the heart rate and to a dilation of the vessel system. This constitutes measures which are adapted to rapidly stabilize blood pressure. Since these regulations work in both directions, a constant levelling around a specific blood pressure takes place. This regulation process which also takes place at rest becomes apparent in longwave (about 0.1 Hz) blood pressure and heart rate variations.
To measure the baroreflex, a method is usually used in which the blood pressure is increased or reduced by external measures (tilt table, pharmacologically, external pressure onto the carotid arteries, etc.) and the blood pressure and the heart rate are thus continuously measured. The heart rate is then plotted against the blood pressure (one respective value for each heartbeat), and a straight line is interpolated through the correspondingly activated region. The baroreflex sensitivity (BRS) in ms/mmHg then results from the slope.
This method is problematic in that the measurement is in most cases invasive (injection of a medication bolus) and in that further regulation parameters can be changed by the medication or the blood pressure activation itself.
Alternatively, the measurement of the longwave rest regulation may be suitable. Here, a data record for the further analysis is obtained via a continuous ECG and a continuous blood pressure measurement over a period of up to several hours. For this time, the patient is at rest such that only the auto-regulative mechanisms are to be visible. Corresponding correlations between the blood pressure and the heart rate variability can be derived from the variation with time of blood pressure and heartbeat interval values.
A known method to this end is the so-called sequence method. Sequences of at least three consecutive increasing (or decreasing) blood pressure values are here searched for in the blood pressure series. Correspondingly decreasing (increasing) sequences to these sequences are searched for in the heart rate values, either beginning at the same heartbeat or shifted by one or several heartbeats, depending on the model taken as a basis. Slopes are in turn calculated from these associated sequences and are averaged and indicated as BRS values.
The difficulty of this measurement consists in the determination of these sequences. At rest, the underlying regulation mechanisms are poorly pronounced, and the blood pressure variations are low. They can therefore easily be superimposed by noise and are thus not detected. This leads to long measurement times or to statistically not meaningful mean values.
There are further correlation methods in the frequency range of the measured values or on the basis of other correlation parameters between the blood pressure curve and the heart rate, which are however considerably more complicated with regard to calculation and interpolation.
The drawback in the known correlation-based methods is that though the underlying regulation mechanisms have a relatively constant wavelength at rest, they have no coherent phase due to physiological processes. Owing to movement, respiration, cough or similar processes, these regulations may have short “blackouts” of a few heartbeats, which complicates a normal frequency-based correlation analysis.
All aforementioned methods have in common that they require a continuous blood pressure measurement. The continuous heart rate measurement is carried out in an unproblematic manner via a normal ECG lead and the determination of the R waves in the ECG. The heartbeat interval is defined via the time interval of the R waves.
The continuous blood pressure measurement takes place in a non-invasive way, usually by finger cuffs in which the cuff pressure is maintained exactly on the pulse pressure in the finger by a regulation circuit. This is measured by an IR diode which records the pulsatile change in volume. This method has the drawback that due to the constriction on the finger, it is uncomfortable for the patient and can thus not be used over longer time periods. Furthermore, the handling of the finger cuff is laborious due to pressure hoses and additional height correction sensors.
Furthermore, a regular calibration is necessary for the determination of the blood pressure from the cuff pressure. During this calibration, the cuffs are inflated up to a maximum value and released again in intervals during several seconds such that a conventional oscillatory blood pressure measurement can be carried out for calibration. The are no continuous blood pressure data for this period of time, which means that the latter cannot contribute to the measuring. In an extreme case, the blood pressure itself may even be influenced by the intense inflation of the cuff.
The bivariate phase-rectified signal averaging=BPRSA is known from the article “Bivariate phase-rectified signal averaging—a novel technique for cross-correlation analysis in noisy nonstationary signals”, Journal of Electrocardiology 42 (2009), 602-606, A. Bauer et al. On the one hand, this method describes a so-called phase-rectified averaging in which the (physiological) signals, the phase of which can be interrupted, can again be made available by an appropriate synchronized averaging of the frequency analysis. Furthermore, the ratio between two different synchronously recorded biosignals is observed to describe corresponding correlations. However, the method described here also has the aforementioned drawbacks with regard to the measurement of the blood pressure.